The present invention relates to an electronic watch and more particularly to an electronic wristwatch which comprises an improved level setting circuit. 2. Description of the Prior Art:
Electronic wristwatches and other small portable time keeping devices are well known. In FIG. 1, an example of a general electronic wristwatch is shown in block diagram. A clock pulse with predetermined frequency is produced at oscillator 10 and applied to frequency divider 11. The output pulse from the frequency divider 11 is applied to a seconds counter 12, in which the counting of the second pulse is made producing a minute pulse to be supplied to the minutes counter 13. In a similar manner, minutes counter 13, hours counter 14 and days counter 15 operate and the respective counter output is applied to selection circuit 16 to select one or some outputs of the counters, and the selected output is applied to the decode circuit 17, in which the count output is transformed into a segment signal that is applied to drive circuit 18. The drive circuit 18 drives display 19 such as a Liquid Crystal Display panel.
In an electronic watch of this type, time correction and the selective display of time are carried out using switches 20 and 21, level setting circuit 22 and control circuit 23. First switch 20 and second switch 21 are connected to first level setting circuit 24 and second level setting circuit 25 respectively, and the outputs of the level setting circuits are supplied to control circuit 23. The control circuit 23 sends out a time correction signal to the counters for changing the count content thereof, corresponding to the operation of first switch 20. On the other hand, corresponding to operation of the second switch 21, one or some count outputs are selected in the selection circuit 16 and displayed at display 19 by way of decode circuit 17 and drive circuit.
In FIG. 2, a prior art level setting circuit is shown. An inverter circuit 26 is connected to switch 20, and an N channel type MOS transistor 27 is connected between the input portion of the inverter circuit 26 and a power source V.sub.ss as a resistive means. The level setting circuit 24 outputs two level states.
Generally, the above-mentioned wristwatch system is formed as a module and further packaged in a metal package for shielding external noise. But if it is difficult to shield the module for some reason, the prior art circuit suffers from many disadvantages. Namely, if the resistance value of the resistive means is large, the input impedance seen from the switch becomes large. Accordingly, external noise is apt to enter into the wristwatch, causing the wristwatch to malfunction. On the other hand, to lower the input impedance, namely to prevent the malfunction, it might be proposed to lower the resistance of the MOS transistor 27, but in this case the current flowing in the MOS transistor 27 will become large. That is to say, power consumption becomes large, which shortens the battery life.
As mentioned above, it is difficult to select an ideal resistance value for the resistive means.